1. Field of the Invention
The present invention relates to an elevator speed control system.
2. Description of the Related Art
Referring to FIG. 7, the configuration of a conventional elevator speed control system will be described. FIG. 7 shows a conventional elevator speed control system, as described, for example, in Japanese Patent Application Laid-Open No. 3-23180.
The conventional elevator speed control system shown in FIG. 7 includes: a contact 2 of an electromagnetic contactor connected to a three-phase AC power supply 1; contacts 3A and 3B of an electromagnetic contactor connected to the contact 2; a reactors 4A and 4B connected to the contacts 3A and 3B, respectively; converters 5A and 5B connected to the reactors 4A and 4B, respectively; smoothing capacitors 6A and 6B for smoothing output voltages given by the converters 5A and 5B; contacts 7A and 7B of an electromagnetic contactor for connecting the output voltages of the converters 5A and 5B in parallel; inverters 8A and 8B connected to the smoothing capacitors 6A and 6B, respectively; reactors 9A and 9B provided so as to connect to outputs of the inverters 8A and 8B, respectively; contacts 10A and 10B of an electromagnetic contactor connected to the reactors 9A and 9B, respectively.
FIG. 7 also shows an induction motor 11 connected to the contacts 10A and 10B, a speed reducer 12 connected to the induction motor 11, a sheave 13 of a hoisting machine connected to the speed reducer 12, a main rope 14 wound around the sheave 13, a car 15 connected to the main rope 14, and a counterweight 16.
The conventional elevator speed control system further includes a control circuit 17 connected to the converters 5A and 5B, and also includes a control circuit 18 connected to the inverter 8A and 8B.
Now, operation of the conventional elevator speed control system will be described. When an elevator is given an operation instruction, the electromagnetic contactor is excited to close the contacts 2, 3A, and 3B, thus the three-phase AC power supply 1 is connected to the converters 5A and 5B via the reactors 4A and 4B. Using PWM modulation, the converters 5A and 5B control the voltages across the capacitors 6A and 6B to maintain a certain DC voltage. The contacts 7A and 7B of the electromagnetic contactor are also closed so that the capacitors 6A and 6B are connected in parallel with each other and so that converters 5A and 5B operate in parallel.
Using PWM modulation, the inverters 8A and 8B, operating in parallel, convert the DC voltage to an AC voltage whose magnitude and frequency are controllable. Then, the converted AC voltage is supplied to the induction motor 11 via the reactors 9A and 9B, and via the contacts 10A and 10B. The output of the induction motor 11 is reduced by the speed reducer 12 to move the car 15 and the counterweight 16 by rotating the sheave 13.
In the conventional elevator speed control system as in the above description, when parallel operation is carried out via reactors between the converter 5A and converter 5B or between the inverter 8A and inverter 8B, circulating current flows through the parallel-connected converters or inverters, thus causing a reduction in output.